Sarawak is the largest state of Malaysia and it's over 600 km away from the mainland. The distances are large in Sarawak. The roads are good and it runs quiet, but there are only two lanes (without much traffic). You misjudged you as you travel quickly from Kuching to Kota Kinabalu. Only the distance to the border at Miri lasts 14 hours, if you are travelling by express bus. But the scenery is beautiful. You get the opportunity to make beautiful pictures.

The long-distances buses stop regularly. None of the buses are equipped with toilets but there is a scheduled stop every three hours somewhere you can buy snacks and use the toilet. Be sure to wear an extra jumper or bring a blanket, as the air-conditioning on these buses is heavy.
On the page "Bus Terminal Sarawak " you can check how long a journey by bus takes. A journey from Kuching to Sibu can you better make do by boat than by road. The travel time is significantly shorter. In Sibu you can travel along by bus, if you want. Compared to Kuala Lumpur, the buses drive more quiet here.
There are several Low Budget Airlines in Malaysia, three of which are operating in Borneo: AirAsia , MASwings and Malindo. MASwings lands at most airports. They fly you to your destination for little money.
However, if you want to get around in Sarawak, than you can use this distance chart.